Integrated circuits are used in the vast majority of modern electronic devices. An integrated circuit is formed by fabricating devices on/in a wafer. The wafer contains at least one die (also referred to as a chip), which in turn contains an integrated circuit. The integrated circuit, in general, may provide multiple functions. The circuitry that provides each of these functions often is sensitive to electrical signals generated from other nearby circuitry of the integrated circuit. Accordingly, the sensitive circuits are isolated from other circuits. Thus, isolation is becoming increasingly important due to higher levels of integration (which increases the noise between circuits), higher frequencies used to communicate signals in the integrated circuit (which increases the coupling between the circuits), and tighter specifications for new devices.
Integrated circuits are typically isolated, for example, by fabricating the circuits relatively far apart on the die from the other circuits. The circuits are disposed relatively far from each other to increase the resistance between the circuits and thereby better isolate the circuits. The resistance is characteristic of the substrate in which the circuits are fabricated.
In some fabrication processes, however, a grounded metallized ring (edge seal) contacting the substrate is placed around the outside of the die to seal the edge from contaminating ions that affect the yield during processing and affect performance of the chip after the chip has been fabricated. In addition, the edge seal is grounded to the substrate.
Accordingly, the edge seal acts as a low-impedance path between different regions on the die. This means that isolation between circuits in these regions is limited by the edge seal, which effectively shorts out a substantial amount of the electrical connection through the substrate. For example, if two circuits are located near the edges of the substrate and are 2000 microns apart, the resistance between the grounds may be several hundred ohms. If an edge seal is placed around the perimeter of the die 10 microns from each circuit, the effective distance between the circuits is now only 20 microns apart. This consequently decreases the isolation of the resistance by a factor of 100 due to the low impedance metal connection around the die.
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